The present invention relates generally to a stator for a motor, and more particularly to an externally-wound stator with improved magnetic transition.
FIG. 6 is a perspective view of a known internally-wound stator core 10. The internally-wound stator core 10 includes a number of circumferentially spaced-apart pole pieces or teeth 12 which cooperate to defined radially inwardly opening slots 14 which receive coils of wire during a stator winding operation. The stator core 10 can be wound in one of two known techniques, either pre-winding the wire into coils and then inserting the coils radially outwardly into the slots 14 from a central bore 16 of the stator core 10, or using a needle to thread or wind the wire around the pole pieces 12 from within the slots 14.
A disadvantage associated with pre-winding a set of coils is that the wires that form the coils are susceptible to damage such as nicking and/or scraping the insulated coating that surrounds or encases the wires, when the coils are inserted around the pole pieces 12 and into the slots 14 from within the central bore 16. Damaged coil wires can electrically short to other coil wires thus reducing the wire turn count and causing excessive heat generation which may, inter alia, shorten the lifetime of the stator. A disadvantage associated with the needle winding technique is that there are limitations on the number of wire coils (the amount of wire fill) that can be wound around the pole pieces 12 because a certain amount of free space must be reserved in the slot 14 to permit the needle to enter and feed the next wire through the slot 14.
FIG. 7 is a perspective view of a known externally-wound stator core 20 which eliminates the potential for damaging the coil wires, and eliminates the wire fill limitation associated with the internally-wound stator core 10. The externally-wound stator core 20 includes a thin circular member 22 defining an inner diameter of the stator core 20, and a number of circumferentially spaced-apart pole pieces or teeth 24 which are individually joined to the member 22 at the radially innermost edges thereof. The purpose for the member 22 is to structurally retain the pole pieces 24 in a predetermined position so that the stator core 20 can be wound with coils of wire during a stator winding operation. The pole pieces 24 cooperate to defined radially outwardly opening slots 26 for receiving the coils of wire which are wound around the pole pieces 24 in a manner similar to winding an armature. It should be appreciated that a winding operation for an externally-wound stator core is faster and simpler than a winding operation for an internally-wound stator core.
One disadvantage of joining the radially innermost edges of the pole pieces 24 to the member 22 is that the sharp magnetic corners of the pole pieces 24 at the inner diameter of the stator core 20 are eliminated or at least substantially diminished. That is, the member 22 provides a continuous inner circumferential surface which does not permit abrupt changes in reluctance. Sharp magnetic transitions at the radially innermost edges of the pole pieces are required for variable reluctance motor technologies such as switched reluctance motors. However, the member 22, which is a structural feature of the stator core 20, provides a magnetic short circuit which may cause a significant amount of flux leakage when a predetermined set of coils is energized. With particular types of magnetic motors, the leakage is an acceptable consequence of the member 22. However, with variable reluctance motor technologies, the flux leakage is not acceptable as it directly affects torque production.
What is needed therefore is an externally-windable stator core which preserves the sharp magnetic corners of the pole pieces at the inner diameter of the stator core.